Most of us have had those annoying tunes or jingles that get stuck in our heads on a playback loop, sometimes running for hours or even days.
The instrument for these "earworms" is the auditory cortex, the part of the brain that passes information from the ears to the sensory system and plays a role in creating musical memories.
Typically, people are able to flush the obsession by listening to other music or focusing on another task, perhaps a physical workout, researchers at the University of Cincinnati have found.
For some people, though, the playback goes beyond one song to a whole repertoire of musical hallucinations that go on and on.
Experts consider it a form of tinnitus -- the perception of sound without any actual external sound. Usually this involves buzzing or ringing, but in this case, the distracting phantom sound is music.
Musical hallucinations are vivid, and often seem to involve songs that are religious or spiritual in nature. Women experience the hallucinations more often than men. They're particularly prone if they're elderly and have experienced hearing loss. Some cases have been documented in younger people who have had a reaction to drugs or other types of hearing damage.
At least one British researcher who studied hallucinatory patients found several years ago that the patients didn't use the main auditory cortex to experience phantom sounds, but a more advanced region of the brain that converts simple sounds into complex music. Some researchers think the hallucinations happen as the brain tries to replace sensory input that's otherwise been lost.
One music hallucination hallmark has been that the tunes patients replay are always songs they've heard before, often many times, and seem to carry emotional significance.
However, that may not always be so. Researchers at Loyola University of Chicago Medical Center describe the experience of a hearing-impaired woman, age 60, who suddenly began hallucinating music as she was falling asleep. She couldn't identify the tunes she sang or hummed, but her husband recognized them as popular tunes. The researchers said it was notable that the woman was able to "play back" the music even though she didn't recognize it, suggesting that she may have known the tunes at one time but forgotten them in her conscious mind.
The case may help reveal new information about the mechanisms of forgetfulness, Danilo Vitorovic and Jose Biller write in the journal Frontiers in Neurology. "In other words, is forgotten information lost, or just not accessible?" the doctors questioned.
Perhaps more bizarre is the experience of visual musical hallucination. That is, some people "see" written musical scores, often very ornate, when nothing similar is actually before them.
Dr. Oliver Sacks, a neurology professor at New York University, has been studying these visions of musical notation and described the experience of eight patients in the July issue of the journal Brain.
He noted that the hallucinations can occur in connection with a number of medical conditions, including Parkinson's disease, fever, intoxication and visual impairment -- and that all but one of the patients were gifted musicians.
One, a surgeon and amateur pianist who was losing his vision to macular degeneration, started seeing musical notations he said appeared "just like a sheet of real music."
Another patient with Parkinson's reported seeing staves and lines of music on the pages of books. Sacks said it's not clear what parts of the brain are involved in the hallucinations, but speculates they are regions involved in reading words as well as music.
All eight patients reported a common feature of the imagined music scores: discordance.
While the musical scores they imagined were often intricate, when they considered them closely they realized actually playing them would create anything but sweet music and no familiar tunes.
Much of the time, the intricate scores were, in the words of the surgeon, "unreadable and unplayable" or would have created simply a cacophony of tones.